三分法模板

double ternarySearch(double l,double r)
{
    while(r-l < eps)
    {
        double mid = (l + r)/2;
        double midmid = (mid + r)/2;

        if(cal(mid) < cal(midmid))
        {
            l = mid;
        }
        else
        {
            r = midmid;
        }
    }
    return l;
}